Multifuel heat generator with integrated circulating bed

ABSTRACT

Heat generator comprising a combustion chamber, a circulating bed and a recovery boiler. The circulating bed and the combustion chamber have a common wall. The present invention may be used to achieve the combustion of high sulphur content fuels.

BACKGROUND OF THE INVENTION

The subject of the invention is a heat generator able to burnhigh-sulfur fuels and which, in the form of a compact assembly, enablesthe production of useful heat to be separated from the desulfurizationof the flue gases.

Strict regulations in protected zones govern emissions of sulfur oxidesin the gaseous effluents from heat generators and forbid the use ofhigh-sulfur fuels which, however, have definite economic advantages:this is the case for certain coals related to lignites, and for oilresidues from refining processes.

Aside from downstream processes for treating fumes which generally applyto very-high-power facilities, in certain fossil-fuel-burning thermalunits desulfurization is accomplished in the course of combustion bydirectly injecting a calcium-based absorbent (limestone, lime, dolomite,etc.) into the hearth.

This in situ desulfurizing process is considered principally for solidfuels, and its efficiency (between 30 and 60%) is a great contributor tothe temperature distribution in the combustion chamber, while requiringsubstantial lime consumption (Ca/S ratio=Calcium/Sulfur ratio on theorder of 3 to 4 moles/mole).

A different method consists of using so-called "dry ash" fluidized-bedboilers which operate at about 800°-900° C. and in which fuel andabsorbent are placed in intimate contact.

In particular, within a "fast" or "circulating" fluidized bed havingsystematic recirculation of the solid particles, a very high rate ofdesulfurization can be obtained (85-90%) with relatively modest Ca/Sratios (1.5 to 2 moles/mole).

However, the self-desulfurizing circulating-bed heat generator poses anumber of technological problems.

In particular, its reliability is closely linked to the strength ofheat-exchanging tube bundles and to abrasion and corrosion phenomena.

The device proposed has the essential advantage of being reliable sinceit can be implemented by using tested techniques. Moreover, thegenerator according to the present invention is compact and takes upvery little space.

The basic idea is based on the combination of three principal elementsarranged such that the exchange surfaces are protected from the rapidflow of solid particles which are often the cause of rapid deteriorationof these surfaces.

Thus, the generator proposed consists essentially of a hearth orcombustion chamber, preferably with cold walls, a recovery boilercapturing the sensible heat of the flue gases, and an intermediatecirculating bed with an insignificant internal exchange surface, whosefunction is to desulfurize the gases passing between the hearth upstreamand the exchanger downstream.

"Cold wall" is understood herein to mean that the wall has means forextracting heat.

In general, the present invention relates to a great generator with acombustion chamber, a circulating bed, and a recovery boiler. Accordingto the present invention, the circulating bed and combustion chamberhave a common wall.

This common wall may have at least one orifice for feeding into thecirculating bed a stream of primary fluid and/or at least one orificefor feeding into the circulating bed a stream of secondary fluid.

This common wall may be a cold wall. Likewise, other walls of thecombustion chamber may be cold walls.

The various cold walls may have provision for circulation of a fluid.

According to the present invention, the circulating bed and the recoveryboiler may have a common wall.

Likewise, the combustion chamber and the recovery boiler may have acommon wall.

The walls of the circulating bed may have a coating made of aheat-insulating material.

The desulfurizing circulating bed whose entrained solid material isessentially, the absorbent, uses the hot gases coming from the hearth asa working fluid.

Since the temperature of the gases may vary with the generator load, thebed may be maintained at the optimum desulfurizing temperature(800°-900° C.) by injecting a makeup fuel into the reactor, wherebycombustion takes place with the excess oxygen from the upstream hearth,possibly with additional fresh fuel.

The compactness of the generator according to the invention is achievedby original spatial distribution of the three main elements disposedvertically. This compactness facilitates its prefabrication. The presentinvention will be better understood and its advantages will emerge moreclearly from the description hereinbelow of particular non-limitativeexamples illustrated by the attached figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the general layout of the heat generator according tothe invention,

FIG. 2 is a simplified perspective view of such a heat generator, and

FIGS. 3 and 4 show two particular versions of the arrangement of thevarious elements of the heat generator.

DETAILED DESCRIPTION

The principle of the compact self-desulfurizing unit according to theinvention is illustrated by FIG. 1, which shows a particular exampleadapted to combustion of a solid or liquid fuel injected in the powderform into the upstream hearth or combustion chamber.

Combustion chamber or hearth 1 is preferably cold-walled, wherebyexchange surfaces 2 are for example of the "diaphragm wall" type, i.e.the fluid circulating means are associated and/or integrated with thewalls of the combustion chamber. These cold walls are sized such thatthe temperature of the combustion gases at the end of the hearth may bein the range of 600°-850° C. for all operating modes.

Burner 3 may advantageously be a "low NO_(x) emitting" burner to limitnitrogen oxide emissions and contribute to making the generatorcompletely non-polluting.

Under these conditions, the excess air or excess fuel can be regulatedsuch that the quantity of residual oxygen is at least equal to thatnecessary to effect the second combustion in circulating bed 16, whichhas a reactor 6 and a separator 10 which may be of the cyclone type.

Reactor 6 of circulating bed 16 is joined to hearth 1 by a common wall17, communication between these two elements being accomplished directlyby one or more passages provided in this wall. The stream 41 of primarygas supplying the circulating bed and coming from combustion chamber 1enters through lower passage 4, while the stream of secondary gas entersthrough upper passage 5.

The internal walls 7 are made of a layer of refractory insulatingmaterial which may be thin abrasion-resistant and the heat losses areessentially recovered by the heat-conducting fluid which bathes thejacket of hearth 1.

In the example of FIG. 1, auxiliary fuel and/or the material absorbingsulfur oxides is/are injected through at least one orifice 9 in thelower part of reactor 6, which is the dense phase of the circulatingbed. However, it will not be a departure from the scope of the inventionto inject these products at another point in the circulating loop of thecirculating bed, particularly by injecting one or both products intoreturn leg 20.

The oxidizing gases or fumes 41 and 51 coming from the lower 4 and upper5 passages defined above and serving as a working fluid and comburantfor the circulating bed are injected on either side of the dense phase18 of this bed.

The gases or fumes in primary stream 41 penetrate into dense phase 18via a perforated grid 8 or any other device ensuring good distributionof the gases throughout the fluidized solids.

The gases or fumes in secondary stream 51 are injected into thetransition zone or diluted zone of reactor 19, also known as the releasezone. They may also be distributed through several orifices in astraight cross section or stepped cross sections relative to thecirculating axis in reactor 6. The same applies to the introduction ofthe primary stream.

Controlled distribution by appropriate means such as fume flaps betweenprimary stream 41 and secondary stream 51 allows the progress ofcombustion in reactor 6 to be controlled and the flow of solids sweptoutside dense zone 18 to be sent to recycling.

This recycling is effected by means of separator 10 which canconveniently be a cyclone as stated above. The recirculation rate isgoverned by a valve device 12 which may be of mechanical or hydraulicdesign, for example a fluidized siphon or "L valve."

The assembly of reactor 6, cyclone 10, and link leg 20, whichconstitutes desulfurizing circulating bed 16, is heat-protected byrefractory insulating coatings 7 and 11.

The desulfurized gases 21 leave the upper part of separator 10 to feedrecovery boiler 13 and give up heat energy to exchange surfaces 14 whichmay be made of tube bundles.

The fumes are finally evacuated via pipe 15 and sent to the filtrationsystem not shown in the diagram, which may be of a type known to theindividual skilled in the art.

The solid waste which has not been recycled or which has escapedseparator 10 of circulating bed 16 may be drawn off at the bottom of thecombustion chamber through orifice 22 which may be blocked by a valve23, at the bottom of dense phase 18 of the circulating bed at the levelof grid 8, through orifice 24 which may have a valve 25, and/or thebottom of the recovery boiler through orifice 26 which may be blocked byvalve 27.

In the embodiment shown in FIG. 1 which relates to production ofnon-superheated steam, the heat-conducting fluid 28 such as awater-steam emulsion coming from the combustion chamber is sent to apressurized container or tank 29 through a line 30. This tank, locatedat the top of the generator in the example of FIG. 1, also receives inthis example water-steam emulsion 28a coming from recovery boiler 13through line 30. The fluid stored in container 29 is transferred in theform of steam via a line 31 to a consumer system such as a turbine 32, aheating system, etc. The heat-conducting fluid, after giving up part ofits energy and after condensation in a condenser not shown, isdistributed by a valve means 33 between the heat-conducting fluid feedto tube bundles 14 of recovery boiler 13 and the heat-conducting fluidfeed of the circuit bathing combustion chamber 1, whereby said circuitmay have pipes forming an integral part of the walls of this combustionchamber or may be formed by a sheet of water.

The heat-conducting fluid is carried between the outlet of turbine 32and valve 33 and the feed to tube bundles 14 and pipe 34 by pipes 35,36, and 37 shown at least partially in dot-dashed lines. Of course,these pipes can be heat-insulated.

FIG. 2 shows an example of the practical implementation of a unitwherein optimum compactness has been achieved by setting hearth 1,reactor 6 of circulating bed 16, and recovery boiler 13 edge-to-edge.

The straight sections of these component parts are rectangular (see FIG.3), which allows them to have close thermal contact with each other andminimizes fatal losses from the walls to the surrounding environment.

In FIG. 2, wall 17 is interrupted before reaching the lower part 38 ofhearth 1 and the reactor of circulating bed 6, thus allowing simplecreation of lower passage 4.

This figure does not show the cyclone, the heat-conducting fluidcirculating pipes, or the burner.

Reference 39 designates the orifice allowing burner 3 to be set in place(FIG. 1).

Orifice 40 designates the outlet orifice from reactor 6 of thecirculating flow 42 proceeding toward separator 10.

Reference 43 designates the inlet orifice for gases 21 coming fromseparator 10 and proceeding toward recovery boiler 13 (FIG. 1).

In the embodiment shown in FIG. 2, circulating bed 6 is not extendedheightwise in the same way as hearth 1, but is interrupted in the frontby wall 44. The latter is surmounted by a parallelepipedic casing 45 indirect communication with recovery boiler 13 which is alsoparallelepipedic in shape.

Orifice 46 corresponds to the link of leg 20 (FIG. 1) connectingseparator 10 (FIG. 1) to the reactor of circulating bed 6 (FIG. 1).

FIG. 3 represents a cross section at the level of the reactor of thecirculating bed of the generator shown in FIG. 2.

In this FIG. 3 we see that reactor 6 of circulating bed 16 is thermallyisolated on its four faces by the material designated by reference 47.The combustion chamber has a plane wall 48 common to both reactor 6 ofthe circulating bed at 49 and to the recovery boiler at 50.

Recovery boiler 13 and reactor 6 of the circulating bed have a commonwall 52 which is substantially perpendicular to plane wall 48.

FIG. 4 represents an alternate version of the device according to theinvention wherein it is the recovery boiler 13 which has a plane wall 53common to both hearth 1 and reactor 6 of the circulating bed.

Reference 54 designates the wall common to hearth 1 and reactor 6,whereby this wall can be substantially perpendicular to plane wall 53 ofthe boiler.

In FIG. 1, valve 33 can be controlled bearing in mind the power demandfrom turbine 32, the quantity of fuel consumed by burner 3, and/or thetemperature of reactor 6 of the circulating bed.

Introduction of an auxiliary fuel into the circulating bed at 9 forexample, although not essential, permits more flexible control of thetemperature of the circulating bed.

We claim:
 1. Heat generator comprising a combustion chamber, acirculating bed, and a recovery boiler, a common wall means disposedbetween said circulating said bed and said combustion chamber, at leastone first passage means provided in said common wall means for supplyingprimary gas from the combustion chamber to the circulating bed, and atleast one second passage means disposed in said common wall means at aposition higher than said first passage means for supplying a stream ofsecondary gas, whereby oxidizing gases from said first and secondpassage means serve as a working fluid and comburent for the circulatingbed.
 2. A heat generator according to claim 1, wherein said common wallmeans includes a cold wall.
 3. A heat generator according to claim 2,wherein said combustion chamber includes a cold-wall.
 4. Generatoraccording to one of claims 2 or 3, wherein said cold walls includesmeans for circulating a fluid.
 5. Generator according to claim 3,wherein a common wall means is provided between said circulating bed andsaid recovery boiler.
 6. Generator according to claim 5, wherein commonwall means are provided between said combustion chamber and saidrecovery boiler.
 7. Generator according to claim 6, wherein walls of thecirculating bed have a coating made of a heat-insulating material. 8.Generator according to claim 1, wherein a common walls means is providedbetween said circulating bed and said recovery boiler.
 9. Generatoraccording to one of claims 1 or 8, wherein common wall means areprovided between said combustion chamber and said recovery boiler.